Secondary database is not joined
This policy checks the join state of the secondary database (also known as a "secondary database replica"). The policy is in an unhealthy state when the dataset replica is not joined. The policy is otherwise in a healthy state.
For this release of SQL Server 2014, information about possible causes and solutions is located at Secondary database is not joined on the TechNet Wiki.
Use Transact-SQL, PowerShell, or SQL Server Management Studio to join the secondary replica to the availability group. For more information about joining secondary replicas to availability groups, see Joining a Secondary Replica to an Availability Group (SQL Server).